Ions, or charged atomic particles, can be confined and suspended in free space using electromagnetic fields. Quantum bits (“qubits”) are stored in stable electronic states of each ion, and quantum information can be processed and transferred through the collective quantized motion of the ions in the trap (e.g., interacting through the Coulomb force). Lasers are applied to induce coupling between the qubit states (e.g., for single qubit operations) or coupling between the internal qubit states and the external motional states (e.g., for entanglement between qubits).
Current ion trap research is largely driven by the quest to construct a quantum information processor, where qubits of information are stored in individual atomic ions and connected through a common interaction with a phonon or photon field. The fundamental experimental requirements for quantum processing have all been met with ion traps, including demonstrations of multi-qubit quantum gates and small algorithms.
The fundamental operations of a quantum computer have been demonstrated experimentally with high accuracy (or “high fidelity” in quantum computing language) in trapped ion systems, and a strategy has been developed for scaling the system to arbitrarily large number of qubits by shuttling ions in an array of ion traps. This makes trapped ion systems one of the most promising architectures for a scalable, universal quantum information processor, as well as for the development of miniature mass spectrometer arrays, compact atomic clocks. Work on miniaturizing electromagnetic traps to the micrometer scale promises even higher levels of control and reliability.
The considerations with respect to the construction and operation of a quantum information processor include the following.
The first consideration concerns qubits. Any two-level quantum system can form a qubit, and there are two ways to form a qubit using the electronic states of an ion. First, the use of two ground state hyperfine levels (these are called “hyperfine qubits”). Second, the use of a ground state level and an excited level (these are called the “optical qubits”). Hyperfine qubits are extremely long-lived (e.g., decay time on the order of thousands to millions of years) and phase/frequency stable (e.g., have traditionally been used for atomic frequency standards). Optical qubits are also relatively long-lived (e.g., decay time on the order of a second) compared to the logic gate operation time (e.g., on the order of microseconds). However, each type of qubit poses its own challenges in the laboratory.
The second consideration concerns initialization. Ions can be prepared in a specific qubit state using a process called optical pumping. A laser couples the ion to some excited states which eventually decays to one state which is not coupled to by the laser. Once the ion reaches that state, it has no excited levels to couple to in the presence of that laser and therefore remains in that state. If the ion somehow decays to one of the other states, the laser will continue to excite the ion until it decays to the state that does not interact with the laser. This initialization process is standard in many physics experiments and can be performed with extremely high fidelity (e.g., >99.9%).
The third consideration concerns measurement. Measuring the state of the qubit stored in an ion is quite simple. Typically, a laser is applied to the ion that couples only one of the qubit state. When the ion is collapsed into this state during the measurement process, the laser will excite it, resulting in a photon being released when the ion decays from the excited state. After decay, the ion is continually excited by the laser and repeatedly emitting photons. These photons can be collected by a photomultiplier tube (“PMT”) or a charge-coupled device (“CCD”) camera. If the ion is collapsed into the other qubit state, then it does not interact with the laser and no photon will be emitted. By counting the collected photons, it is easy to determine which state the ion is in with very high accuracy (e.g., >99.9%).
The fourth consideration concerns arbitrary single qubit rotation. One of the requirements of universal quantum computing is to coherently change the state of a single qubit. For example, this can transform a qubit starting out in 0 into any arbitrary superposition of 0 and 1 defined by the user. In trapped ion systems, this is often done using magnetic dipole transitions or stimulated Raman transitions for hyperfine qubits, and electric quadrupole transitions for optical qubits. Gate fidelity can be as high as >99%.
The fifth consideration concerns two qubit entangling gates. Besides the controlled-NOT gate, many equivalent but more robust schemes have been proposed and implemented experimentally. Recent theoretical work has shown that there are no fundamental limitations to the speed of entangling gates, but gates in this impulsive regime (e.g., faster than 1 microsecond) have not yet been demonstrated experimentally (e.g., current gate time is on the order of microseconds). The fidelity of these implementations have been as high as >97%.
The sixth consideration concerns scalable trap designs. Efforts in this area are now focused on the scaling of ion traps to host much larger numbers of qubits, perhaps by shuttling individual atoms through a complex maze of ion trap electrodes. For example, several groups have successfully fabricated ion traps with multiple trap regions and have shuttled ions between different trap zones. Thus, ions can be separated from the same interaction region to individual storage regions and brought back together without losing the quantum information stored in their internal states. Ions can also be made to turn corners at a “T” junction, allowing a two dimensional trap array design.
Although ions traps present a great deal of promise, especially with respect to quantum information processing and computing, they still must overcome significant manufacturing and scalability issues that have greatly limited their overall use and effectiveness.
Accordingly, there exists a need for new and improved ion traps and methods for making the same, especially those that are suitable for use in conjunction with quantum information processing and computing.